Step 1: Divide the number into separate digits:
8651 => 8 6 5 1
Step 2: Calculate the distance between each digit and the next (the last digit with the first):
8 to 6 = 2
6 to 5 = 1
5 to 1 = 4
1 to 8 = 7
Step 3: Combine the filters to a string:
2 1 4 7 = 2147